1. Field of the Invention
The present invention belongs to a technical field of an image display device and in particular a medical image display device for displaying an image photographed by a medical diagnostic apparatus or the like. More particularly, the present invention relates to a medical image display device capable of displaying a plurality of medical images arranged in a row as visible images.
2. Description of the Related Art
A medical image photographed (measured) by a medical diagnostic apparatus such as an ultrasonic diagnostic apparatus, a CT diagnostic apparatus, an MRI diagnostic apparatus, an X-ray diagnostic apparatus, or a CR apparatus including an FCR (Fuji computed radiography) is subjected to various image processing operations as required. After that, generally, the medical image is reproduced as a visible image on a film-shaped recording material by a printer such as a laser printer or a thermal printer and outputted as a hard copy.
On a medical site, the film on which the medical image is reproduced is observed using a light box called Schaukasten, and is used for various diagnoses.
Also, in recent years, the diagnosis has been conducted by reproducing as a soft copy the medical image photographed by the medical diagnostic apparatus on a display device such as a CRT (cathode ray tube) or a liquid crystal display. In addition, such a diagnosis has come to be conducted, in which a work station for diagnosis provided with a CRT is connected to the medical diagnostic apparatus through a network, and the diagnosis is conducted through observing the photographed medical image in a consulting room or the like which is located apart from the medical diagnostic apparatus.
Also, more recently, a thin type liquid crystal display can attain high quality image display. As a result, the liquid crystal display has been increasingly used instead of the CRT in various display fields. In the liquid crystal display, light emitted from a backlight is made incident on a liquid crystal, and a voltage applied to the liquid crystal is changed for each corresponding pixel in accordance with image data. Thus, the transmittance of light passing through the liquid crystal is changed to display an image on a surface side thereof.
In particular, in a liquid crystal display for medical use including medical diagnosis, a cold cathode fluorescent lamp (CCFL) is used as a backlight. The CCFL functions on the principle that the luminance can be controlled based on the adjustment of a ramp current value. In addition, for the CCFL, there are a direct type structure and an edge light structure.
A plurality of shapes are used for the direct type CCFL, which may be a straight tube, a U-shaped tube, a W-shaped tube, or the like. The direct type CCFL is located immediately under a display region surface of an array substrate in which a liquid crystal is sealed and has a structure in which a liquid crystal in the display region is directly irradiated with light. Note that a light curtain and a diffusing plate are attached onto the upper surface of the direct type CCFL to reduce luminance unevenness.
According to the direct type CCFL, because the liquid crystal in the display region is directly irradiated with light, the amount of light can be efficiently utilized, so that high luminance can be achieved easily. Therefore, such a structure is the mainstream particularly in image display on a film viewer (so-called electronic film viewer) used in the medical field that requires high luminance. In other words, a liquid crystal display on which the direct type CCFL is mounted is more appropriate for obtaining high luminance than a liquid crystal display on which an edge light type CCFL to be described later is mounted. Therefore, the former is very often used for the film viewer in the medical field.
On the other hand, for the edge light type CCFL, a straight tube, a U-shaped tube, an L-shaped tube, or the like is used. The edge light type CCFL is located in the edge of a light guide plate opposed to the liquid crystal in the display region and has a structure in which the liquid crystal in the display region is indirectly irradiated with light through the light guide plate. Note that a reflector for condensing reflected light to the light guide plate is disposed near the CCFL. In addition, a reflecting sheet for reflecting incident light from the edge to the liquid crystal in the display region is provided in the light guide plate.
A liquid crystal display on which the edge light type CCFL is mounted can also be used for the film viewer (electronic film viewer) in the medical field by increasing the luminance of light emitted from the CCFL itself.
Here, in order to conduct further accurate diagnosis in medical examination and treatment using the medical diagnostic apparatus, it is general that a large number of images are photographed for each diagnosis while changing a photographing condition, a photographing angle, and a photographing region. In general diagnosis using the film viewer, a plurality of films on which the thus photographed medical images are reproduced, are arranged on the film viewer and the diagnosis is conducted through observing and comparing the respective images.
However, when a plurality of medical images photographed by the medical diagnostic apparatus are displayed on a plurality of CRTs or liquid crystal displays to conduct the diagnosis, it is general that one image is displayed on the screen of a CRT or the screen of a liquid crystal display. Accordingly, the luminance must be adjusted for each CRT or each liquid crystal display.
In particular, in a liquid crystal display on which the direct type CCFL is mounted, a plurality of light emitting tubes are disposed for the liquid crystal in the display region. Therefore, if the amounts of light of the respective light emitting tubes are not uniform, luminance unevenness is caused. The thus caused luminance unevenness is a serious problem leading to a false diagnosis particularly in the medical field, so that the luminance unevenness should be rapidly compensated for. In addition, in any of the direct type CCFL and the edge light type CCFL, the reduction in luminance causes a problem in that an image is hard to view and so on, which leads to a false diagnosis. Accordingly, it is necessary to keep adequately high luminance.
The luminance unevenness has been conventionally adjusted by measurement using a luminance meter for each case. This is inconvenient because a luminance meter is necessary for each case. Therefore, a hole is formed on the rear side of the liquid crystal display to locate an optical sensor so that the luminance of a backlight may be measured. Alternatively, an optical sensor is located in the front side of the liquid crystal display, with which the luminance may be measured.
However, when the hole is formed on the rear side of the liquid crystal display, the hole causes the luminance unevenness. In addition, when the optical sensor is located on the front side of the liquid crystal display, a space for the optical sensor is necessary, which causes a problem in that a screen view is blocked.
In the case of a diagnosis using an image reproduced on a film, the image is fixed on the film and hence the diagnosis can be basically conducted by observing the single image, although certain differences may occur due to the luminance of a film viewer used or observation environment.
On the other hand, when conducting the diagnosis using an image displayed on a display device such as a CRT or a liquid crystal display, the displayed image, that is, the diagnostic image is changed by the type, state and aging of a display device used, because it is image data that is fixed thereon. Such a difference in the image causes a serious problem which may lead to a false diagnosis. Therefore, the quality control (QC) for appropriately retaining the state of a display device used is important when conducting diagnosis using the display device.
Various test items including observation condition, (luminance) gradation properties, spatial resolution and geometry are set for the quality control of display devices. The important measurement for conducting these tests is the measurement of luminance such as maximum luminance and minimum luminance of a display device, and surface reflection luminance of a display device in association with peripheral light.
An exterior luminance meter must be used to measure the luminance, which is laborious. In particular, the high-accuracy luminance measurement requires the use of a telescopic luminance meter, which is however expensive and is not easy to handle. A contact type luminance meter is known as the meter capable of simple luminance measurement but luminance cannot be appropriately measured in the contact type luminance meter because a displayed image is changed when a force is applied to the image display screen of a liquid crystal display, which is well known in the art.
Further, it is necessary to measure the surface reflection luminance under observation conditions using a telescopic luminance meter when quality control is performed according to DICOM (Digital Imaging and Communications in Medicine; transmission standard of medical image data, waveform data and the like), which makes the quality control work of a display device complicated.